<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>两列布局的第一种解决方案</title>
  <style>
    #left,
    #right {
      height: 300px;
    }

    #left {
      /* 定宽 */
      width: 400px;
      background-color: #c9394a;
      /* 当前元素脱离文档流 */
      float: left;
    }

    #right {
      background-color: #cccccc;

      margin-left: 400px;
    }

    #inner {
      height: 300px;
      background-color: green;

      clear: both;
    }
  </style>
</head>

<body>
  <div id="left"></div>
  <div id="right">
    <div id="inner"></div>
  </div>
</body>

</html>